The Creepy Treat Cards are predominantly gained from fight sequences, Bad Guys will always drop 1 card, sometimes 2 if it's a bigger group you're fighting.
A couple cards are awarded by completing the Apple Bobbing challenges as well as some are "Rare" cards you can trade duplicates for with kids in all three areas. These kids will have a card over there head once you talk to them. In most cases you will have the card the require already if you have been doing alot of fighting.
In your menu you can go to your Creepy Treats page to see which Cards your missing. Page 1 is your neighborhood, page 2 is the mall, and page 3 is the carnival. If you are missing any cards, head back to that area and look for a kid to trade with, or bad guys to fight. I got my last card in the corn maze.
EDIT: The official website has a list and picture of all the cards
http://www.costumequestgame.com/notebook.aspx#treats